This invention relates to in situ recovery of oil from tar sand located in a subsurface formation. More particularly, the invention provides a method for recovering oil from a subsurface tar sand formation by means of a unique steam injection system.
There are many methods for recovering a resource, such as tar sand, from beneath the earth surface. Where there is little overburden, surface mining techniques have been widely employed. However, when the overburden is thick or the ratio of overburden to tar sand formation thickness is high, then surface mining is no longer economical. Many in situ recovery methods have been proposed over the years. Typically, wells are drilled from the earth surface down into the tar sand formation. These tar sand formations in their normal or undisturbed state are very viscous and immobile. Many different techniques have been developed to establish both a communication path through the heavy, highly viscous bitumen-filled sand and an efficient method to recovery the bitumen from the sand. These methods include such things as steam injection, solvent flooding, gas injection, etc. Such processes generally involve the heating of the tar sand formation to reduce the viscosity of the formation, thereby allowing removal of the resource from the formation in flowable form by hydraulic means or gravity flow.
U.S. Pat. No. 4,160,481 uses a plurality of bore holes radially extending from a central bore hole to inject steam into the tar sand formation. Steam is injected into some bore holes to drive the oil into the remaining bore hole where it is collected.
In Turk et al., U.S. Pat. No. 4,160,481, a method is described in which perforated radial tubes extend laterally into the formation from a central bore hole. That system uses a cyclic steam injection procedure. After a number of steam injection/production cycles, the process can be converted to a continuous steam drive where steam is continuously injected into one radial and oil is produced from another radial.
Bouck et al., U.S. Pat. No. 4,463,988, describes an in situ recovery system for a tar sand deposit in which a network of horizontal production tunnels and connecting horizontal bore holes are provided. This is a complex structure and a difficult and expensive one to install and operate.
Bielstein et al., U.S. Pat. No. 3,386,508, describes a system for recovering oil in which a plurality of directional (slant) wells are drilled from the surface to intersect a central vertical well within an oil bearing formation. Both the directional wells and the vertical well bore communicate fluidly with the oil bearing formation.
In Renard et al., U.S. Pat. No. 5,016,710, another system for recovering oil is described having a plurality of slant wells drilled from the surface to cooperate with a central vertical well within an oil bearing formation. With this design, steam may be injected into the oil bearing formation either from the central vertical well or from the plurality of slant wells.
It is an object of the present invention to provide an improved system for recovering normally immobile hydrocarbon oil from a subsurface tar sand formation by steam injection.